Numerous different constructions for multi-coordinate probes, both of the switching and of the measuring kind, have been proposed in the past. In both types of probes, it is of decisive importance that the stylus, after having been deflected in any of the directions in which it can move, is reliably returned into its zero position. For this purpose, a variety of constructions have been attempted in which the mounting or seating of the stylus relative to the probe casing is designed in a manner intended to accomplish this purpose.
One of those constructions is disclosed in German Offenlegungsschrift No. 28 41 424. This publication is concerned with a probe wherein the stylus has a three point seating. Three V-grooves are arranged in star formation in the bottom portion of the probe casing. Three pins with spherical surface are fixed in a mounting plate for the stylus. The mounting plate is urged by a spring in the direction of the probe casing bottom, while the pins assume in the associated V-grooves a predetermined position into which they are supposed to return after deflection of the stylus.
Another construction of this type is disclosed in European Application No. A2-0088596 which also teaches a seating or bearing arrangement at three locations. In this prior art proposal, seating locations are distributed in the same plane and uniformly over the circumference. Three balls are arranged in the bottom of the probe casing. These balls, for assembly purposes, are movable in direction of the stylus. The mounting plate has three counter bearings or abutments of different design. One of the counter bearings serves the purpose of preventing rotation of the plate and is in the form of a bore. The second counter bearing has a fixed V-groove which is directed towards the center point of the mounting plate. Finally, the third counter bearing is a plane member which extends parallel to the mounting plate and perpendicular to the force component of the compression spring.
It is generally known in this art that measuring force differentials and the bending-dependent switching path differentials associated therewith, are significantly reduced in different probing direction, by increasing the number of mounting locations for the stylus. However, in the known probe systems, an increase in the number of seating locations also significantly increases the expenditure necessary for manufacturing the mounting locations with the required precision fit. This in turn creates the danger that the syste becomes statically overdefined and stable equilibrium conditions are lost. This is so because even in the manufacture of seatings with precision fit, certain manufacturing tolerances cannot be avoided, resulting in static instability.
The prior art as embodied by EP-2A-0088596 attempts to avoid static overdefinition or underdefinition by providing the different designs at the three seating locations.
Seating locations of different design are thus necessary in both prior art systems, if more than three such locations are to be provided. While this results in high zero position stability for the stylus, the different construction for the seating locations has also a distinct disadvantage. This is so, because when the stylus ball probes an object and dependent on the probing direction, a deflection of the stylus in different preferred directions is probable.